The present invention relates to optical fiber transmission systems and more particularly to an improved configuration for launching a single mode long wavelength transmission signal into existing multimode optical fiber networks.
Much of the installed based of optical fiber consists of multimode fiber. The problem addressed by this invention is that a significant amount of the installed multimode fiber base consists of inferior quality fiber where the bandwidth associated with the Over-Fill Launch (OFL) condition is not realized. In fact, approximately 10% of installed fiber can suffer bandwidth collapse with long wavelength laser transmitters when launched on axis or with a typical offset of 3 to 7 um for a multimode fiber connector. This has been a serious problem in the past, and has been the focus of many experts in this area for the past 15 years. As early as 1991, the US Patent to Yoshizawa U.S. Pat. No. 5,077,815 disclosed the general concept of launching the transmission signal down the multimode fiber offset from the optical axis of the core. Yoshizawa utilized a single mode optical fiber coupled to a laser diode, or LED, for launching the transmission signal into the core of the multimode fiber. The single mode fiber was butt-coupled to the end face of the multimode fiber with the core of the singe mode fiber offset from the optical axis of the multimode fiber. Alternate configurations using lenses to focus the optical radiation from the single mode fiber to the multimode fiber were also disclosed. The theoretical basis for the realized improvement in bandwidth was believed to be a preferential excitation of higher order mode groups as opposed to lower order mode groups.
In 1995 the US Patent to Haas No. U.S. Pat. No. 5,416,862 identified that adding an angle to either a center launch or an offset launch would further preferentially excite higher order mode groups traveling in the multimode fiber and would also increase bandwidth.
The most recent work in this area can be found in the US Patent to Cunningham et al U.S. Pat. No. 6,064,786 which describes a theoretical model that distinguishes preferential excitation of only mid-order modes. The primary configurations claimed in Cunningham use a long wavelength (1300 nm) Fabry Perot edge emitter diode, and require the use of a multimode collecting fiber for collecting optical radiation from the diode. Low order mode groups are excited in the collecting multimode fiber and are launched into a conducting multimode fiber offset from the optical axis of the conducting multimode fiber where they preferentially excite mid-order mode groups. One alternate configuration disclosed in the Cunningham '768 patent briefly describes the experimental testing of a short wavelength (850 nm) VCSEL operating in a single transverse mode and launching the VCSEL radiation into the multimode fiber offset from the optical axis of the core. However, practical use of the system was discounted due to power limitations of the VCSEL. The short wavelength VCSEL described in Cunningham would had to have been operated at a low power in order for it to lase in only a single transverse mode. For the most part, the '768 Cunningham patent seems to set forth an explanation of the theoretical reasons “why” offset launch provides better bandwidth.
Another practical solution for providing an offset launch into multimode fiber communication systems has been addressed by the GbE standards bodies by specifying a mode conditioning patchcord that is used to guarantee a known offset in the range 17 to 23 um for 62.5 um multimode fiber and 10 to 16 um for 50 um multimode fiber where the reduced number of excited modes guarantees satisfactory performance. The mode conditioning patchcord is an extra component used between a conventional single mode fiber transmitter (transceiver) and the multimode fiber system that guarantees the optimum launch condition. Further details of the multimode fiber patch cord configurations can be found in the Cunningham U.S. Pat. No. 6,304,352.
A further improved solution to this problem has been found and is the basis for the present invention. The fiber bandwidth demonstrated for the present invention is far better than predicted, and this has opened up a new commercial opportunity for data links on multimode fiber by extending the distances and data transfer speeds that can be accommodated.